Rubber compositions for tire treads which are excellent in a wet skid performance (herein after referred to as a WET performance) related to a running stability on a wet road surface have so far been proposed. It has so far been known as techniques for enhancing a WET performance that rubber is blended with silica in a high proportion, that a glass transition temperature (Tg) of rubber, that is, 0° C. tan δ is raised and that carbon black which is crushed into pieces having a finer particle size is blended in a high proportion. However, rubber compositions for tire treads which are blended with silica in a high proportion have a problem on a workability (processability). Rubber compositions in which Tg of rubber is elevated have problems on a reduction in a low temperature performance and a rise in a rolling resistance (herein after referred to as ┌RRJ┘). Rubber compositions in which carbon black crushed into pieces having a finer particle size is blended in a high proportion are elevated in RR.
Accordingly, a rubber composition for a tire tread which is improved in a WET performance by specific silica and a technique for kneading and a production process for the same are proposed. For example, 100 parts by weight of natural rubber and/or diene base synthetic rubber is blended with 10 to 80 parts by weight of a low temperature plasticizer having a freezing point of −48° C. or lower and 5 to 40 parts by weight of silicon carbide, silicon nitride, aluminum oxide or silica each having an average particle diameter of 0.1 to 1 mm or a mixture thereof to intend to enhance the WET performance (refer to, for example, a patent document 1). However, the problems of the workability (processability), the abrasion resistance and the low-heat property (the property with little generation of heat) remain in the above improvement.
Developments for enhancing a braking and driving performance (herein after referred to as an on-ice performance) of a tire on an ice and snow road surface have been carried out since spike tires were regulated. A water film is liable to be produced on a snow and ice road surface due to frictional heat of the snow and ice road surface with a tire. The water film reduces a frictional coefficient between the tire and the snow and ice road surface. Accordingly, a water film-removing ability on a tread of a tire, an edge effect and a spike effect exert an effect on an on-ice performance to a large extent. A water film-removing ability, an edge effect and a spike effect of a tread have to be improved in order to enhance an on-ice performance in a tire.
In order to provide a tread with a water film-removing ability, a large number of micro drain ditches (both depth and width: about 100 μm) are provided on the surface of a tire to remove a water film by these drain ditches, and a frictional coefficient of the tire on a snow and ice road surface is increased. In the above case, an on-ice performance of the tire in the beginning of use can be enhanced. However, the problem that the on-ice performance is slowly reduced as the tire is abraded is involved therein. Then, it is consider to form bubbles in a tread in order to prevent the on-ice performance from being reduced when the tire is abraded.
On the other hand, described is a method in which the micro drain ditches described above are formed on the surface of a tread by using foamed rubber blended with short fibers for the tire tread (refer to, for example, a patent document 2). In the above case, the short fibers do not readily break away from the tread when the tread is abraded by running. However, the short fibers are not approximately parallel to the abraded face in many cases, and micro drain ditches which are aimed from the beginning can not always efficiently be formed. Accordingly, the frictional coefficient on a snow and ice road surface has not sufficiently been improved. Further, there has been observed the problem that break away of the short fibers depends on running conditions and the like to a large extent to make it impossible to surely enhance the on-ice performance.
Further, it is proposed that in vulcanizing a rubber composition containing a foaming agent, organic fibers in which a viscosity is lowered than that of the composition until the temperature reaches a vulcanization maximum temperature are blended to carry out vulcanization (refer to, for example, a patent document 3). In the above case, there is the effect that micro drain ditches are formed on the surface of a tread tire to improve a water film-removing ability. However, in the case of the above tread tire of vulcanized rubber, there is room for improvement as far as a rise in an edge effect and a spike effect (scratching effect) is concerned.
Further, it is proposed to provide organic fibers with various functionalities (refer to, for example, a patent document 4). For example, it is proposed that fine particle-containing organic fibers prepared by adding fine particles having a prescribed diameter such as glass fine particles, aluminum hydroxide fine particles, alumina fine particles, iron fine particles, (meth)acrylic resin fine particles, epoxy resin fine particles and the like to organic fibers having a prescribed diameter and molding them are added to a rubber component of a tire. Further, in a rubber composition for a tire, fine particle-containing organic fibers in which a viscosity of a fiber resin is lowered more than that of the rubber matrix until a temperature of the rubber composition reaches a vulcanization maximum temperature in vulcanization are used in order to enhance a water film-removing ability and an edge effect in the tire. The water film-removing ability, an effect for elevating the frictional coefficient and the edge effect are improved by applying the fine particle-containing organic fibers. However, the above organic fibers containing fine particles have to be increased in a diameter thereof to some extent. Accordingly, this causes a reduction in the extruded skin and brings about a reduction in the plant workability, and in addition thereto, it exerts an adverse effect on a foamed layer forming micro drain ditches in extrusion.
Patent document 1: Japanese Patent Application Laid-Open No. 135241/1990
Patent document 2: Japanese Patent Application Laid-Open No. 38207/1992
Patent document 3: Japanese Patent Application Laid-Open No. 48264/1999
Patent document 4: Japanese Patent Application Laid-Open No. 233993/2001